MPTCP is missing from system parameters inside Mininet hosts

[mrconter1]

Hello,

I am having problems with certain system parameters not being accessible from within a Mininet host even though they are accessible from the host system.

After a clean install with the following:

• Ubuntu 20.04.5 LTS
• The 5.4.230.mptcp kernel

I get the following output when running sysctl -a | grep mptcp outside Mininet:

net.mptcp.mptcp_checksum = 1
net.mptcp.mptcp_debug = 0
net.mptcp.mptcp_enabled = 1
net.mptcp.mptcp_path_manager = fullmesh
net.mptcp.mptcp_scheduler = default
net.mptcp.mptcp_syn_retries = 3
net.mptcp.mptcp_version = 0

which is correct. But when I do the same from within a Mininet host by starting Mininet with the default topology:

sudo mn

and then running the sysctl command on, for instance, the h1 node:

mininet> h1 sysctl -a | grep mptcp

I dont get any output. In other words, the system parameters can't be found by sysctl -a from within a Mininet host.

Expected/Desired Behavior:

mininet> h1 sysctl -a | grep mptcp

=>

net.mptcp.mptcp_checksum = 1
net.mptcp.mptcp_debug = 0
net.mptcp.mptcp_enabled = 1
net.mptcp.mptcp_path_manager = fullmesh
net.mptcp.mptcp_scheduler = default
net.mptcp.mptcp_syn_retries = 3
net.mptcp.mptcp_version = 0

Actual Behavior:

mininet> h1 sysctl -a | grep mptcp

=>

[No output]

Detailed Steps to Reproduce the Behavior

1. Install "Ubuntu 20.04.5 LTS" and the "5.4.230.mptcp" kernel.
2. Run sudo mn
3. Inside the Mininet console execute sysctl -a | grep mptcp
4. Compare output when running it outside the host

Additional Information

The following people seems to have had the same problem:

• TR : proc/sys/net/mptcp missing
• mptcp inside lxc container can't access /proc/sys/net/mptcp_enabled

---

Best regards, Rasmus

[matttbe]

Hi Rasmus,

I don't know how mininet works these days: does it create network namespaces per host or is it a VM?

Because (out of tree) MPTCP's sysctl knobs are not net namespace aware: you set them on the "host" (the initial netns) but not per netns: they are not visible from the netns. Like other sysctls.

If you don't know how to check, maybe try this from a "host" and check if it is not empty

# ip netns identify $$

Or simply check if you have MPTCP this file from a "host": /proc/net/mptcp_net/mptcp

(or check nsenter, compare readlink /proc/self/ns/net, check if qemu is running, etc.)

[mrconter1]

Thanks for your reply. Mininet works by running the hosts in different namespaces and they aren't separate VM's. Here are some outputs from both the host system and from within a Mininet host. It seems to be the same output regardless of if sudo or not is used in both cases.

Command	Result in Ubuntu (outside a Mininet host)	Result from within a Mininet host
ip netns identify $$	[No output]	[No output]
ls /proc/net/mptcp_net/	mptcp snmp	mptcp snmp
readlink /proc/self/ns/net	net:[4026532008]	net:[4026533846] (differs between Mininet hosts)
ls /proc/sys/net/mptcp	mptcp_checksum mptcp_debug mptcp_enabled mptcp_path_manager mptcp_scheduler mptcp_syn_retries mptcp_version	[No output]

I think it's relevant to note that creating hosts with inNamespace=False:

h1 = net.addHost('h1', inNamespace=False) # don't spawn in net namespaces
h2 = net.addHost('h2', inNamespace=False) # don't spawn in net namespaces
r1 = net.addHost('r1', inNamespace=False) # don't spawn in net namespaces

results in me being able to see the system parameters from inside a Mininet host by running sysctl -a | grep mptcp. While this solves the access problem it seems to result in the network parameters that you can set for each Mininet host to be ignored.

I think I somehow need to figure out how I can mount the namespace or perhaps the "/proc/sys/net/mptcp" folder. Running mount --bind /proc/sys/net/mptcp from within a Mininet host just results it:

mount: /proc/sys/net/mptcp: mount point does not exist.

[matttbe]

OK, good to know.

Why do you need to access the sysctl entries from the namespaces? If you change them, they will change the behaviour in all the netns.

[mrconter1]

Each Mininet host needs to be able to access the system parameters in order to work properly.

[matttbe]

I don't think you can make them appearing in /proc/sys/net/mptcp without breaking stuff (or changing the kernel code to expose the parameters to all netns or make them netns aware).

Can you not dump the parameters in a file and read this file?

Or maybe you can try something like as root from a netns:

mkdir -p /var/run/netns/
ln -sfT /proc/1/ns/net /var/run/netns/host

then from the netns, you might be able to do:

ip netns exec host sysctl net.mptcp

[matttbe]

or make them netns aware

Probably too late to do that for this out-of-tree kernel. Maybe time to switch to the upstream one? https://mptcp.dev

[mrconter1]

I don't think you can make them appearing in /proc/sys/net/mptcp without breaking stuff (or changing the kernel code to expose the parameters to all netns or make them netns aware).

Can you not dump the parameters in a file and read this file?

Or maybe you can try something like as root from a netns:

mkdir -p /var/run/netns/
ln -sfT /proc/1/ns/net /var/run/netns/host

then from the netns, you might be able to do:

ip netns exec host sysctl net.mptcp

When I do the following from within a Mininet host:

sudo mkdir -p /var/run/netns/
sudo ln -sfT /proc/1/ns/net /var/run/netns/host
sudo ip netns exec host sysctl net.mptcp

The last command fails with:

mount of /sys failed: Device or resource busy

[mrconter1]

or make them netns aware

Probably too late to do that for this out-of-tree kernel. Maybe time to switch to the upstream one? https://mptcp.dev

That might be the way forward. I am also thinking of trying this out from Debian. That might work better but I am just speculating...

[matttbe]

Yes, there might be a way but I never had to do that (and it is not specific to MPTCP).

I don't think using Debian will change anything but why not trying.
Maybe also look at nsenter, I guess mininet uses that.

(or maybe they still have a way to use different VMs? I thought it was like that in the past, a long time ago)

[matttbe]

Maybe this? (probably you will get the same error)

nsenter -t 1 sysctl net.mptcp

[mrconter1]

Unfortunately it doesn't seem to work either:

mininet> h1 nsenter -t 1 sysctl net.mptcp
sysctl: cannot stat /proc/sys/net/mptcp: No such file or directory

But anyways. Thanks for the help! I will try my luck with Debian now.